Conventional ultrasonic transducers are described in numerous publications. Many applications, such as ultrasonic sensors and proximity switches, require ultrasonic transducers having a very narrow sound cone characteristic while at the same time having very small secondary cone amplitudes.
German Patent No. 25 41 492 describes a very sharp ultrasonic transducer with saturn geometry, which is widely used with ultrasonic sensors in industrial applications. It is constructed from a piezoceramic disk, an adapting layer disk and a concentric metal ring. The area of the adapting layer is large because the adapting layer disk projects a great distance beyond the diameter of the piezoceramic disk, thus yielding a very narrow angle .phi. of the sound cone of 5.degree.. Due to the large projecting area of the adapting layer disk, it is difficult to always accurately maintain the favorable vibrational amplitude distribution and phase angle over the entire area in practice, thus resulting in problematic lateral secondary cones.
German Patent No. 42 33 256 describes an ultrasonic transducer with a round piezoceramic disk in radial resonance, having a reduced operating frequency in comparison with the radial resonant frequency of the piezoceramic disk for given dimensions of the piezoceramic disk. This is achieved using a metal ring surrounding the piezoceramic disk with a form fit and frictional engagement, preferably being shrunk-fit onto the piezoceramic disk. The composite body formed by the piezoceramic disk and the metal ring forms a radial oscillator which behaves like a piezoelectric body with a larger diameter and a reduced frequency with regard to its vibrational properties. By appropriate dimensioning of the metal ring, the operating frequency can be reduced to different extents. An adapting layer is applied to the end face of the composite body.